Time encoded magnetic resonance imaging

Abstract

A method and apparatus for time encoded magnetic resonance imaging is presented. In accordance with the present invention, a static magnetic field oriented in a first direction is generated, with the sample to be imaged disposed within this static magnetic field, whereby the magnetization vector created by atomic particles is constant in size and direction as a result of the static magnetic field. The sample is then exposed to a first linear magnetic field gradient oriented in a second direction to define a slice. The sample is then excited with a radio frequency (RF) pulse, at the resonance frequency, the pulse being oriented in a third direction perpendicular to the first direction. This causes the magnetization vector of the atomic particles to rotate 90° into a plane perpendicular to the first direction. The sample is then exposed to a two dimensional, non-uniform, magnetic field gradient oriented in a plane perpendicular to the first direction. The sample is then exposed to a second uniform magnetic field gradient oriented in a fourth direction perpendicular to the second direction. A spin echo is produced by pulse or gradient reversal then the RF signals are detected. RF signals are emitted as a result of the spin echo, these RF signals vary in time as a result of the exposure to the two dimensional, non-uniform, gradient magnetic field. This time variance is indicative of spacial position within the slice. The RF signals received as a function of time are converted to a set of frequency domain functions at specific times relating to specific strips in the image by, for example, Short Time Fourier Transformer (STFT). These frequency domain functions in the form of strips, are combined sequentially to form the entire time-frequency domain function or the image.

What is claimed is: 
     
       1. A method for magnetic resonance imaging of a sample, comprising the steps of: generating a static magnetic field orientated in a first direction, the sample being disposed within said static magnetic field, whereby a magnetization vector of a plurality of atomic particles of the sample align with said static magnetic field in said first direction;   generating a first generally linear magnetic field gradient oriented in a second direction, the sample being exposed to said first linear magnetic field gradient to define a two dimensional slice of the sample in a first plane generally perpendicular to said second direction;   generating a first radio frequency pulse at a resonance frequency of the atomic particles, said first radio frequency pulse oriented in a third direction, said third direction being generally perpendicular to said first direction, the sample being exposed to said first radio frequency pulse, said first radio frequency pulse having a first time duration wherein said magnetization vector is rotated into a second plane generally perpendicular to said first direction;   generating a two dimensional, non-uniform, magnetic field gradient oriented in said second plane, the sample being exposed to said two dimensional, non-uniform, magnetic field gradient at a time other than when said sample is exposed to said first radio frequency pulse;   generating a spin echo in the sample, whereby radio frequency signals are emitted from the atomic particles at different times which correspond to two dimensional spacial positions in said slice;   generating a second generally linear magnetic field gradient oriented in a fourth direction, said fourth direction being generally perpendicular to said second direction, the sample being exposed to said second linear magnetic field gradient;   detecting said radio frequency signals emitted from the atomic particles to provide a detected signal indicative of a time domain function of said radio frequency signals detected for said two dimensional slice; and   processing said detected signal to transform said detected signal indicative of the time domain function of said radio frequency signals detected to processed signals indicative of a set of frequency domain functions of said radio frequency signals detected.   
     
     
       2. The method of claim 1 wherein said step of generating a spin echo comprises: generating a second radio frequency pulse at the resonance frequency, said second radio frequency pulse oriented in said third direction, said second radio frequency pulse having a second time duration sufficient for rotating said magnetization vector about 180°, the sample being exposed to said second radio frequency pulse.   
     
     
       3. The method of claim 2 wherein said second time duration is about twice as long as said first time duration. 
     
     
       4. The method of claim 1 wherein said step of generating a spin echo comprises: generating a reverse generally linear magnetic field gradient orientated in a direction opposite to said second direction, the sample being exposed to said reverse linear magnetic field, in response to said exposure to said reverse linear magnetic field, said magnetization vector is rotated about 180°.   
     
     
       5. The method of claim 1 wherein said step of processing comprises Fourier transform processing. 
     
     
       6. The method of claim 5 wherein said Fourier transform processing comprises short time Fourier transform processing. 
     
     
       7. The method of claim 1 further comprising the step of: sequentially combining said processed signals to generate an image indicative of said two dimensional slice of said sample.   
     
     
       8. The method of claim 1 wherein said step of generating said static magnetic field further comprises: continuously varying the strength of said static magnetic field over the sample.   
     
     
       9. The method of claim 1 wherein said two dimensional slice is a transverse slice, a sagittal slice or a coronal slice. 
     
     
       10. The method of claim 1 wherein said step of generating said two dimensional, non-uniform, magnetic field gradient comprises generating said two dimensional, non-uniform, magnetic field gradient to replicate in one dimension a spatial variation of said second linear magnetic field gradient.   
     
     
       11. The method of claim 1 wherein said time other than when said sample is exposed to said first radio frequency pulse of said step of generating said two dimension, non-uniform, magnetic field gradient is a time prior to said sample being exposed to said first radio frequency pulse. 
     
     
       12. The method of claim 1 wherein said time other than when said sample is exposed to said first radio frequency pulse of said step of generating said two dimension, non-uniform, magnetic field gradient is a time after said sample was exposed to said first radio frequency pulse. 
     
     
       13. A device for magnetic resonance imaging of a sample, comprising: means for generating a static magnetic field orientated in a first direction, the sample being disposed within said static magnetic field, whereby a magnetization vector of a plurality of atomic particles of the sample align with said static magnetic field in said first direction;   means for generating a first generally linear magnetic field gradient oriented in a second direction, the sample being exposed to said first linear magnetic field gradient to define a two dimensional slice of the sample in a first plane generally perpendicular to said second direction;   means for generating a first radio frequency pulse at a resonance frequency of the atomic particles, said first radio frequency pulse oriented in a third direction, said third direction being generally perpendicular to said first direction, the sample being exposed to said first radio frequency pulse, said first radio frequency pulse having a first time duration wherein said magnetization vector is rotated into a second plane generally perpendicular to said first direction;   means for generating a two dimensional, non-uniform, magnetic field gradient oriented in said second plane, the sample being exposed to said two dimensional, non-uniform, magnetic field gradient at a time other than when said sample is exposed to said first radio frequency pulse;   means for generating a spin echo in the sample, whereby radio frequency signals are emitted from the atomic particles at different times which correspond to two dimensional spatial positions in said slice;   means for generating a second generally linear magnetic field gradient oriented in a fourth direction, said fourth direction being generally perpendicular to said second direction, the sample being exposed to said second linear magnetic field gradient;   means for detecting said radio frequency signals emitted from the atomic particles to provide a detected signal indicative of a time domain function of said radio frequency signals detected for said two dimensional slice; and   means for processing said detected signal to transform said detected signal indicative of the time domain function of said radio frequency signals detected to processed signals indicative of a set of frequency domain functions of said radio frequency signals detected.   
     
     
       14. The device of claim 13 wherein said means for generating a spin echo comprises: means for generating a second radio frequency pulse at the resonance frequency, said second radio frequency pulse oriented in said third direction, said second radio frequency pulse having a second time duration sufficient for rotating said magnetization vector about 180°, the sample being exposed to said second radio frequency pulse.   
     
     
       15. The device of claim 14 wherein said second time duration is about twice as long as said first time duration. 
     
     
       16. The device of claim 14 wherein said means for generating said first and second radio frequency pulses comprise a transmitting radio frequency coil. 
     
     
       17. The device of claim 13 wherein said means for generating a spin echo comprises: means for generating a reverse generally linear magnetic field gradient orientated in a direction opposite to said second direction, the sample being exposed to said reverse linear magnetic field, in response to said exposure to said reverse linear magnetic field, said magnetization vector is rotated about 180°.   
     
     
       18. The device of claim 17 wherein said means for generating said first linear magnetic field gradient and said means for generating said reverse linear magnetic field gradient comprise a plurality of linear gradient coils. 
     
     
       19. The device of claim 13 wherein said means for generating said static magnetic field comprises an electromagnet. 
     
     
       20. The device of claim 13 wherein said means for generating said first linear magnetic field gradient comprises a plurality of linear gradient coils. 
     
     
       21. The device of claim 20 wherein said plurality of linear gradient coils comprise Golay type coils or finger print type coils. 
     
     
       22. The device of claim 13 wherein said means for generating said first radio frequency pulse comprises a transmitting radio frequency coil. 
     
     
       23. The device of claim 22 wherein said transmitting radio frequency coil comprises a Helmholtz type coil or a birdcage type coil. 
     
     
       24. The device of claim 13 wherein said means for generating said two dimensional, non-uniform, magnetic field gradient comprises a plurality of gradient coils. 
     
     
       25. The device of claim 24 wherein each of said plurality of gradient coils comprises an octapole gradient coil arrangement. 
     
     
       26. The device of claim 13 wherein said means for generating a second linear magnetic field gradient comprises a plurality of linear gradient coils. 
     
     
       27. The device of claim 13 wherein said means for detecting said radio frequency signals comprises a receiving radio frequency coil. 
     
     
       28. The device of claim 27 wherein said receiving radio frequency coil comprises a Helmholtz type coil or a birdcage type coil. 
     
     
       29. The device of claim 13 wherein said means for generating said radio frequency pulse and said means for detecting said radio frequency signals comprise a transmitting radio frequency coil. 
     
     
       30. The device of claim 13 wherein said means for processing comprises means for Fourier transform processing. 
     
     
       31. The device of claim 13 further comprising: means for sequentially combining said processed signals to generate an image indicative of said two dimensional slice of said sample.   
     
     
       32. The device of claim 13 wherein said means for generating said static magnetic field further comprises: means for continuously varying the strength of said static magnetic field over the sample.   
     
     
       33. The device of claim 13 wherein said two dimensional slice is a transverse slice, a sagittal slice or a coronal slice. 
     
     
       34. The device of claim 13 wherein said means for generating said two dimensional, non-uniform, magnetic field gradient comprises: means for generating said two dimensional, non-uniform, magnetic field gradient to replicate in one dimension a spatial variation of said second linear magnetic field gradient.   
     
     
       35. The device of claim 13 wherein said time other than when said sample is exposed to said first radio frequency pulse of said means for generating said two dimension, non-uniform, magnetic field gradient is a time prior to said sample being exposed to said first radio frequency pulse. 
     
     
       36. The device of claim 13 wherein said time other than when said sample is exposed to said first radio frequency pulse of said means for generating said two dimension, non-uniform, magnetic field gradient is a time after said sample was exposed to said first radio frequency pulse.